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Alien, Invasive & Pest Species
The prickly pear has been present at these localities for many years, but in the last decade some
unknown environmental factors seem to have favoured the spread of this species, and it is now regarded
as a serious problem and classified as very problematic.
The eradication of the plant from the wild in Gibraltar is seen as an essential step in conserving some
of the Rock’s more important habitats.
Action
The establishment of a programme for the eradication of this species in the wild is a priority. There are
several control methods that have been used widely, especially in Australia and South Africa, where the
species has reached invasive proportions. In these two countries the extent of the invasions has necessitated
the use of biocontrol agents. Several have been identified, tested and used and have proved
amongst the most effective entomological control methods ever used in controlling an invasive species.
Amongst these is the cactus moth, Cactoblastis cactorum (a native of South America) that was introduced
during the 1930s, when the South African Parliament declared prickly pear a national disaster. This species
effectively controls the spread of the wild prickly pear, while allowing the cultivation of the food crop, the
cactus pear. Cactus pear growers regard the moth as a pest, but this species can be controlled by the use
of pesticides. This species was also effectively used in Australia where they were used successfully against
O. ficus-indica and O. stricta (Klein, 2002).
Cochineal insects, sap-sucking insects that feed on the cactus, are also used. The species Dactylopius
opuntiae of the ‘ficus biotype’ is the biocontrol agent of the Prickly Pear (Klein, 2002).
Another two biocontrol agents were tested in South Africa but did not contribute significantly towards
the control of the species, although they did damage the plants. These were the beetles Metamasius spinolae,
which is the most destructive cactus insect in its native range and Archlagocherius funestus, which
attacks only Prickly Pear but caused little damage to the plants.
The Prickly Pear can also be controlled through the application of herbicides. A similar invasive species,
Opuntia stricta, was affecting the Dom António Xavier Pereira Coutinho Nature Reserve in Portugal.
Different approaches were assessed simultaneously and the use of glyphosate directly to the plants
through injection into the cladodes and stem in the summer proved the most effective (Monteiro et al. 2005).
The herbicide Roundup is effective on the Prickly Pear and the makers recommend the use of Roundup
on Opuntia ficus-indica as an application injected into pre-made holes in the stems of trees with 20-250
cladodes, applying 2ml of an 18% solution of the product. Roundup is registered for use against Bugweed,
Bramble, Lantana, Prickly Pear, and Water Hyacinth. It kills all green plants, grasses included. It is usually
applied as a leaf spray at a concentration of 1-2% in clean water. It is not poisonous to animal life and deactivates
upon contact with soil. Roundup can also be used as a 2-4% solution to spay onto freshly cut stumps.
Physical removal of the plant is not effective and for those plants established on cliffs, impossible. The
species germinates easily and sets root from leaf cladodes that have been torn or discarded. Therefore,
the complete removal of the plant is essential to avoid accidental propagation.
In Gibraltar the possibility of the use of the biocontrol agent, the moth Cactoblastis cactorum, should be
investigated. However, the devastating effect that this may have on prickly pear crops in the region must
be taken into consideration, and other methods must be favoured if possible. There are several species of
Opuntia of conservation value growing in the Botanic Gardens and these would be adversely affected by
the moth, although these could be treated against the moth with pesticides.
A serious attempt should be made to treat prickly pear populations with glyphosate herbicides, using
GONHS Cliffs and Caves Section to access those on cliffs.
Further Reading:
• Hernandez, L.R.; Emmel, T.C. (1993) Cactoblastis cactorum in Cuba (Lepidoptera: Pyralidae:
Phycitinae). Tropical Lepidoptera, 4: 45-46.
• Johnson, D.M.; Stiling, P.D. (1996) Host specificity of Cactoblastis cactorum (Lepidoptera:
Pyralidae), an exotic Opuntia-feeding moth, in Florida. Environmental Entomology, 25: 743-748.
• Mahr, D.L. (2001) Cactoblastis cactorum (Lepidoptera: Pyralidae) in North America: a workshop of
assessment and planning. Florida Entomologist, 84: 465-473.
• Mayra Perez-Sandi, C. (2001) Addressing the threat of Cactoblastis cactorum (Lepidoptera:
¨Pyralidae), to Opuntia in Mexico. Florida Entomologist, 84: 499-502.
• Monteiro A., Cheia VM., Vasconcelos T., & Moreira I. (2005) Management of the invasive species
Opuntia stricta in a Botanical Reserve in Portugal. Weed Research 45: 193–201.
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